This invention relates to filters designed to select various particles based on the particles' direction of approach.
Many filters have been designed to select particles based on energy, composition, color or polarization. However, the repertoire of filters that employ directional selection has been quite limited. One class of directional filter allows only those particles traveling at or near a given direction to pass through; all others are absorbed. Other directional filters limit passage to those particles approaching within a specific plane. Other less precise filters limit reception to a region of a hemisphere.
Few techniques presently exist that block particles coming from a single undesirable source while allowing particles from the rest of the environment to pass. One of the few practical methods is the ancient art of holding up an object to block the sun (e.g., a parasol or one's hand). This method, holding an opaque barrier at a distance to intercept undesirable particles incident upon a small detector, has severe limitations. First, its effects are limited to a small area, e.g., a single finger can't shield both eyes. Second, the barrier must be held far enough away from the detector to avoid blocking desirable particles. The net result of these limitations is that the barrier must be larger than the protected area and must be held at a distance equal to the size of the barrier divided by the tangent of the desired angular blocking width. This makes the method unwieldy and imprecise.
Other versions of this method exist. Some sunglasses have dark areas above and below a relatively more transparent horizon area, protecting the wearer against glare from the sky and the ground, while allowing much of the light from the horizon (which generally contains the object of attention) to pass through to the wearer's eyes. Many hats have rims or visors designed to block the sky, and thus the sun, from the wearer's eyes. Many automobiles have tinted windshields, a region at the top being darkened to limit the sunlight striking the driver's eyes, and most have movable opaque visors which can be hand-adjusted to block the setting or rising sun. Venetian blinds can be adjusted to block progressively more light as a light source's angle above the horizon increases. Each of these solutions is either unwieldy or indiscriminate, and none offers a truly satisfactory answer to the problems posed by particle sources which, while limited in size, are nonetheless bothersome because of their intensity.